Micromechanical sensors for measuring acceleration, rotation rate, magnetic field, and pressure, for example, are known and are mass-produced for various applications in the automotive and consumer sectors. Cost-effective manufacture and miniaturization of the components are desirable, in particular in consumer electronics. In particular, an increase in the integration density (i.e., achieving greater functionality in the same installation space) of MEMS sensors is sought. For this purpose, it is increasingly common for a rotation rate sensor and an acceleration sensor to be situated on the same chip.
One of the challenges with combined rotation rate sensors and acceleration sensors lies in the different internal pressures at which the sensors are to be operated. Ideally, a rotation rate sensor is operated with a good vacuum, typically at approximately 0.1 mbar to approximately 2 mbar, while an acceleration sensor should be at least critically damped, and therefore typically operated at internal pressures above approximately 100 mbar. The simultaneous operation of a rotation rate sensor and an acceleration sensor on one chip thus suggests the provision of two separate cavity volumes, having different internal pressures, in a hermetically capped chip.
Various manufacturing methods for setting two cavities having different internal pressures are already known. An example of one suitable method is to apply a getter material in the cavity of the rotation rate sensor, as known from U.S. Pat. No. 8,546,928 B2, for example. Wafer bonding between a MEMS wafer and a cap wafer subsequently takes place at high internal pressure (suitable for the acceleration sensor). After or during the hermetic sealing, the getter is chemically activated via a temperature step and effectuates a greatly reduced internal pressure in the cavity of the rotation rate sensor.
Other methods are so-called reseal techniques, as from US 2010/0028618 A1, for example, in which after the wafer bonding (or closure with the aid of thin film capping technology), one of the cavities is opened, a suitable internal pressure is set, and the cavity is subsequently reclosed. It is possible to either initially close the cavities at low internal pressure and subsequently open the acceleration sensor cavity, provide it with high internal pressure, and subsequently close it, or, after the initial closure at high internal pressure, to open the rotation rate sensor cavity, evacuate it, and close it at low internal pressure. The closure may take place, for example, via thin film deposition, for example oxide or metal deposition, or also via a so-called laser reseal, in which a surrounding area around an access hole, close to the surface, is locally melted by localized heat input with the aid of a laser, thus closing the access hole, as from WO 2015/120939 A1, for example.
Further options for encapsulating different internal pressures are in US 2012/0326248 A1, for example.
Setting suitable internal pressures for acceleration sensors and rotation rate sensors is complicated by the fact that acceleration sensors, which generally have a significantly smaller spring stiffness than rotation rate sensors, and thus also have smaller restoring forces from the mechanical stops, require a so-called anti-stiction coating (ASC) for avoidance of “sticking” or static friction (stiction). The ASC is typically applied prior to the wafer bonding, and forms a Teflon-like monolayer on the silicon surfaces. The undesirable adhesion forces between movable structures and mechanical stops may be greatly reduced in this way. In contrast, for a rotation rate sensor, ASC is not only unnecessary due to the much greater mechanical stiffness, but is even counterproductive for setting a low cavity pressure.
Standard bonding processes such as glass frit bonding or eutectic bonding (between aluminum and germanium, for example) are typically carried out at elevated temperatures higher than 400° C. Capping a rotation rate sensor coated with ASC, using this method, is therefore problematic, since a portion of the ASC molecules have already evaporated from the silicon surfaces and increased the cavity internal pressure.
This problem may be compounded in particular when the cavity volume is very small when a completely flat cap wafer, for example a CMOS wafer with an integrated evaluation circuit, is used, and therefore the particle density of the ASC molecules in the gaseous phase is particularly high. At the same time, the anti-stick properties of the ASC layer in the cavity of the acceleration sensor may deteriorate due to the partial evaporation during the wafer bonding.
In other known closure processes, for example growth of an epitaxial thin film silicon cap, the temperature budget is so high that the deposition of ASC prior to the closure is meaningless, since the ASC molecules essentially completely degrade or evaporate at the high closure temperatures. For the case of silicon thin film capping, the subsequent filling of the sensor with ASC through an access hole and subsequent closure thereof via various deposition processes has therefore been provided, for example, in U.S. Pat. No. 7,221,033 B2.
Patent document DE 10 2014 202 801 A1 provides a method for manufacturing a micromechanical component; from a chronological standpoint, initially a joining process between the MEMS element and the cap element is carried out, and a further processing step for the micromechanical component is carried out only when the high temperature of the joining process no longer prevails. The subsequent further processing step, for example in the form of introducing a defined internal pressure into a cavity, conditioning a surface of MEMS structures, etc., may thus be advantageously carried out at a lower temperature in a more flexible and cost-effective manner.
In such methods, it is also disadvantageous that gases such as H2 or light noble gases such as helium and neon may diffuse through oxide layers and other layers of the MEMS structure at moderate temperatures of approximately 150° C., which may occur in practice. The stated gases may develop during a capping process, due to a chemical reaction in this process, or may diffuse from the sensor wafer or cap wafer due to the high temperature in the capping process. A high internal pressure is set in the cavity of the acceleration sensor. Gases such as N2, which do not diffuse through oxide, are typically used for this purpose. The additional gases which may develop during the capping process and diffuse through oxide constitute only a small portion compared to N2.
If the H2 gas diffuses from the acceleration sensor cavity over the service life of the sensor device, the pressure in the acceleration sensor cavity changes only slightly. Furthermore, acceleration sensors are also insensitive to small pressure changes. However, it may be critical that a portion of the H2 may diffuse into the rotation rate sensor cavity, due to the low internal pressure prevailing there and due to the high sensitivity of the rotation rate sensor to pressure changes, which may result in failure of the rotation rate sensor.